1. Field of the Invention
The present invention relates to data a transmission method in a wireless network, and more a method for transmitting date in a IEEE (Institute of Electrical and Electronics Engineers) 802.11b data transmission wireless local area networks.
2. Description of the Related Art
The IEEE 802.11b standard is a widely spread standard covering a data transmission wireless local area network. The following are some main features of the standard that are necessary for better understanding of the invention claimed.
The IEEE 802.11b physical layer (PHY layer) provides for four transmission rates as 1; 2; 5.5 and 11 Mbps. The PHY characteristics of the IEEE 802.11b standard are entirely determined by dependences of bit error rate on signal-to-noise ratio (SNR).
Data units arriving at the MAC layer (MAC—medium access control) are divided to one or more fragments before transmission. A fragment of a data unit cannot be more than 18432 bits. For transmission of each fragment, a separate MAC protocol data unit (MPDU) is formed. An MPDU 100, shown in FIG. 1, is composed of a MAC header 101, a fragment of a data unit 103 and a frame check sequence 105 (FCS) and is transmitted at one of four transmission rates.
Before each MPDU, a PLCP (physical layer convergence protocol) preamble 107 and a PLCP header 109 having total length of 192 μs are transmitted at 1 Mbps transmission rate The operation of IEEE 802.11b wireless local area networks is based on a carrier sense multiple access with collision avoidance (CSMA/CA). A station having a data unit for transmission shall determine the current state of the medium. If the station identifies that the medium is idle for a time interval equal to a distributed interframe space (DIEFS), the station starts data transmission. If the station identifies that the medium is busy it must defer until the medium is not identified as idle during a time interval equal to DIEFS. Therefore, the station avoids collisions during busy mediums.
Transmission of an MPDU starts from a backoff interval 201, shown in FIG. 2. The beginning of the backoff interval matches for all stations having a data unit for transmission. Each station randomly selects the length of its backoff interval as a random value, uniformly distributed within the setup range. Access to the medium is given to a station that selects the minimal interval 203. It starts transmitting its MPDUs having deffered its backoff interval. The backoff interval of the IEEE 802.11b standard is measured in slots. The minimal interval of FIG. 2 is referred to as a number of idle slots 205 between two successive transmissions in the system.
If the minimal interval (203) of two or more stations matches, they start transmitting their MPDUs simultaneously. In this case collisions occur. During collisions, the probability of receiving the MPDUs is greatly reduced.
Two data transmission mechanisms, namely basic data transmission mechanism and request-to-send/clear-to-send data transmission mechanism, are stipulated for transmission of any MPDU in the IEEE 802.11b standard.
When applying the basic data transmission mechanism the following data transmission sequence, as shown in FIG. 3 is used. A station, which has won the contention, starts transmitting an MPDU 301 directly by the end of its backoff interval 303. In case of successfully receiving an MPDU (301) at the receiving station, the receiving station transmits an acknowledgement 305 (ACK) after a time interval equal to a short interframe space 307 (SIEFS). In the case of successfully receiving the ACK at the transmitting station, the MPDU is successfully received.
When applying the RTS/CTS mechanism, the following data transmission sequence is employed, as shown in FIG. 4. A station that has won the contention, starts transmitting a request-to-send 401 (RTS) by the end of its backoff interval 403. In case of successfully receiving the RTS 401, the receiving station transmits a clear-to-send 405 (CTS) after a time interval equal to the SIFS 407. In the case of successfully receiving the CTS 405, the transmitting station transmits an MPDU 409 after a time interval equal to the SIFS 407. In the case of successfully receiving the MPDU 409 at the receiving station, the receiving station transmits an ACK 411 after a time interval equal to the SIFS 407. In the case of successfully receiving the ACK 411 at the transmitting station, the MPDU 409 is received successfully.
The successful MPDU receiving probability depends on the collision probability in a system determined by the current number of active stations in the system and on the bit error rate (BER) in an MPDU determined by the signal-to-noise ratio (SNR) in a signal of the transmitting station on the receiving station. The successful MPDU receiving probability determines the IEEE 802.11b system throughput. Another factor for determining the throughput is the overhead accompanying the MPDU transmission.
Typical overhead includes: All time intervals when the medium is not busy, consisting of DIFS, Backoff interval (number of idle slots between two successive transmissions in a system) and SIFS; all control messages, consisting of RTS, CTS, and ACK; all control information; consisting of MAC header, FCS, PLCP preamble, and PLCP header. The greater the fragment size, the less the impact of overhead and the greater the MPDU error probability.
To maximize the IEEE 802.11b system throughput, each station can adaptively select a fragment size, transmission rate and transmission mechanism before transmission of each data unit.
A. Kamerman and L. Monteban, “WaveLAN-II: a high-performance wireless LAN for the unlicensed band,” Bell Labs Technical J., pp. 118-133, summer 1997; and Daji Qiao, Sunghyun Choi, and Kang G. Shin, “Goodput analysis and link adaptation for IEEE 802.11a Wireless LANs,” IEEE Trans. Mobile Comput., vol. 1, no. 4, pp. 278-292, October-December 2002 describe the following data transmission method in the IEEE 802.11b system.
In data unit transmission the used size of a fragment is equal to the size of a data unit. Any transmission rate is used in the transmission of the first data unit. A data unit is transmitted. If the data unit is correctly received, the next data unit is transmitted. If the data unit is incorrectly received, it is retransmitted. If two successive transmission attempts of the data unit are unsuccessful, the next more reliable transmission rate is selected as a transmission rate. If ten successive transmission attempts are successful, next less reliable transmission rate is selected as the next transmission rate. The obvious advantage of this method of data transmission in the IEEE 802.11b system is simplicity of implementation. One drawback of the method is the system throughput achieved in its application is significantly less then the maximum obtainable throughput, and that is disclosed in Daji Qiao, Sunghyun Choi, and Kang G. Shin, “Goodput analysis and link adaptation for IEEE 802.11a Wireless LANs,” IEEE Trans. Mobile Comput., vol. 1, no. 4, pp. 278-292, October-December 2002.
According to the prior art disclosure the data transmission method in the IEEE 802.11b wireless local area network that comprises at least one transmitting station and one receiving station consists in the following: a database of dependence of the network throughput on signal-to-noise ratio (SNR), transmission rate and size of a data unit fragment is formed on the transmitting station, before transmitting a data unit, the current SNR value in a signal of the transmitting station is estimated at the transmitting station, those values of fragment size and transmission rate that correspond to the maximal value of the network throughput in the database are selected for the obtained SNR estimate, the selected fragment size is used for transmission of all fragments of a data unit, in transmission of fragments of a data unit, the selected transmission rate is used until a new estimate of the SNR in a signal of the transmitting station is obtained, and then a new transmission rate used in transmission of fragments of a data unit is selected. After obtaining a new estimate of the SNR in a signal of the transmitting station, a new transmission rate corresponding to the maximal value of the network throughput in the database for the selected fragment size is selected.
The prior art has some significant disadvantages. In the prior art, the SNR in a signal of the receiving station is used as an estimate of the SNR in a signal of the transmitting station. This is allowed only in the case of the transmitting and receiving stations are identical (with equal noise figure values) and that transmit at the same power.
The prior art does not consider the current network loading (number of active stations). At the same time, the network throughput and consequently optimal values of the fragment size and transmission rate greatly depend on the current network loading. Since the prior art does not consider this effect, the network throughput is greatly reduced.
The prior art also does not provide for adaptive selection of a data transmission mechanism. Station collision probability increases with an increase in network loading. In such conditions the RTS/CTS mechanism significantly excels the basic data transmission mechanism by the network throughput. As the prior art does not consider this effect, the network throughput is again greatly reduced.